I am trying to cause a particular vgui button to respond to a command by loading a map. I already have my vgui dialog setup, I need to write the OnCommand member function to respond to the command by loading a particular map though I am unable to find out how to do this with the source sdk.

Use clientcmd, instead of server command.
And i believe, you cannot use gamerules, unless you logged into server, or the function is static, so it could be acessed without creating a new gamerules object clientside.